This invention relates to control systems for liquid chromatographs.
Liquid chromatographs may be considered as including a pump system, a pump control system, a chromatographic column, a collecting system and a monitoring system. The pump system includes one or more pumps for supplying solvents to the chromatographic column under the control of the pump control system. The pump control system controls the rate of flow, the pressure and the composition of mixtures of solvents in the solvent stream applied to the chromatograph column. The chromatographic column includes an injector for injecting a sample or samples into the column, column packing and connections for receiving solvents.
Reciprocating piston pumps are popularly used as mobile phase supplies in conventional high performance liquid chromatography. In such systems, flow rates are on the order of 1 to 5 milliliters per minute; pump displacements are on the order of 50 or 100 microliters per stroke; the chromatographic column inside diameters are on the order of 4 millimeters and the volume of effluent detector at the outlet of the chromatographic column is on the order of 8 to 20 microliters. Sample sizes are on the order of 50 microliters.
Micro-scale analytical high performance liquid chromatographs are known. These chromatographs can attain considerably higher sensitivity by using smaller samples on the order of 1 microliter. Internal column diameters are on the order of 1/2 to 1 millimeter and the effluent detector volume may be on the order of 0.3 microliter in such systems.
The conventional liquid chromatography reciprocating pumps have several disadvantages when used in these microsystems, such as: (1) at the required flow rates, which are well under 1 milliliter per minute, there are deleterious effects of pump check valve leakage, pump seal leakage and compression of the working fluid during the reciprocating cycle, resulting in poor flow rate accuracy which makes measurement of retention volumes difficult; and (2) fluctuations in the output pressure and flow rate from these pumps aggravate the already serious problem of noise level in the effluent detector.
Single-stroke, syringe-type pumps do not suffer as much from the flow rate inaccuracy and noise problems of reciprocating pumps, but have the disadavtange of being subject to flow rate errors as a result of temperature changes. These errors occur because the cylinder of the pump contains a large volume of solvent compared to the volume of solvent in the column itself. Normally, the temperature coefficient of expansion of the solvent differs from that of the pump walls and thus, as temperatures change, the volume of the fluid in the chamber changes more than the chamber changes to accommodate the increased volume. Consequently, more or less solvent flows from the pump than the programmed amount.